Absolute Instability near the Band Edge of Traveling-Wave Amplifiers.

Applying the Briggs-Bers "pole-pinch" criterion to the exact transcendental dispersion relation of a dielectric traveling wave tube (TWT), we find that there is no absolute instability regardless of the beam current. We extend this analysis to the circuit band edges of a linear beam TWT by approximating the circuit mode as a hyperbola in the frequency-wave-number (ω-k) plane and consider the weak coupling limit. For an operating mode whose group velocity is in the same direction as the beam mode, we find that the lower band edge is not subjected to absolute instability. At the upper band edge, we find a threshold beam current beyond which absolute instability is excited. The nonexistence of absolute instability in a linear beam TWT and the existence in a gyrotron TWT, both at the lower band edge, is contrasted. The general study given here is applicable to some contemporary TWTs such as metamaterial-based and advanced Smith-Purcell TWTs.

Eustachio and "Libellus de dentibus" the first book devoted to the structure and function of the teeth.

Bartolomeo Eustachio (1520-1574), a major anatomist and physician of the Italian Renaissance, made very significant contributions toward our knowledge of the anatomy and physiology of the dentition, including the first descriptions of the structure of the dental pulp and of the periodontal membrane. His treatise on the teeth, "Libellus de Dentibus", published in 1563, has just been translated into English, and the remarkable descriptions of the dental pulp, the periodontal membrane, the development of both sets of teeth from dental follicles, the trigeminal nerve, and other oral structures have not been fully appreciated. We offer extensive translated material from his treatise to establish the unique insight that Eustachio had into the structure and function of the human dentition, based upon extensive dissections of both human and animal material. Eustachio also had very modern ideas concerning the treatment of periodontitis, including the curettage of granulation tissue to promote reattachment of the gingival tissue.

Optical extraction efficiency in lasers with high Fresnel number confocal unstable resonators.

Using the formulation of Moore and McCarthy for the equation describing the dominant mode in a positive branch confocal unstable resonator in the geometrical optics limit and a simple two-level kinetics model, we analyze the dependence of the optical extraction efficiency (eta) on resonator magnification (M), length (L), small signal gain (go), and (nonsaturable) background absorption (alpha). The model has cylindrical symmetry and spatially uniform small signal gain, absorption, and index of refraction. For fixed gamma= g(0)/alpha and M, a value of g(0)L that maximizes eta is found. For different gamma, the maximum obtainable value of al is found to be independent of M and to depend upon gamma in a simple way.